1. Field of the Invention
The present invention relates to a white balance adjustment apparatus used in video cameras and similar devices.
2. Description of the Prior Art
As the performance of white balance adjustment apparatuses for video cameras has improved in recent years, internal metering devices requiring no external sensors have become the mainstream. White balance control is the process whereby the color temperature of the light source on the subject being imaged by the camera is detected and compensation is applied so that achromatic image areas are recorded achromatically, i.e., so that white is white. One example of this type of white balance adjustment apparatus is described in Japanese patent laid-open publication number 61-184079.
An example of this conventional white balance adjustment apparatus is described below with reference to FIGS. 34 and 35. FIG. 34 is a block diagram of a conventional white balance adjustment apparatus comprising input terminals 1, white detector 2, integrator 3, white balance gain calculator 4, amplifier 5, and output terminals 6. FIG. 35 is an example showing the white detection area of the white detector 2 in this conventional white balance adjustment apparatus. In FIG. 35, the axis of abscissas is the B/G axis, the axis of ordinates is the R/G axis, the dotted line is the axis of color temperature change, and the area contained within the solid line is the white detection area.
The red (R), green (G), and blue (B) video signals input to input terminals 1a, 1b, and 1c are input to the white detector 2. The white detector 2 passes as output signals Rw, Gw, and Bw only those near-achromatic signal components of the input video signal that are within the white detection area shown in FIG. 35. The integrator 3 integrates one field of the near-achromatic video signal output from the white detector 2, and outputs integrated values Rs, Gs, and Bs. The white balance gain calculator 4 then calculates the white balance gain from the output signals Rs, Gs, and Bs from the integrator 3, and adjusts the amplification factor of the amplifier 5. The white-balanced video signals R', G', and B' are then output from the output terminals 6a, 6b, and 6c.
Because the video signals entering the white detection area are not necessarily achromatic, when relatively pale colors near the axis of color temperature change, e.g., white skin tones and greenery outdoors, enter the screen, the conventional apparatus described above falsely determines these color signals to be achromatic, resulting in washed-out colors with white skin tones becoming white.